People are again being urged to be wary of landslides, with torrential downpours expected to cause another round of damaging slips across the North Island.
A coastal erosion expert says rainfall totals forecast with ex-tropical cyclone Gabrielle’s passing – potentially reaching hundreds of millimetres in exposed north-eastern regions – is likely to trigger many more seaside slips, while heavy storm surges could change the shape of beaches.
That comes on top of thousands of landslides caused by the last two major storm systems.
“There is the potential for new landslides to occur, and for existing landslides to reactivate,” Auckland Emergency Management’s Deputy Controller Rachel Kelleher told a media conference today.
She encouraged residents worried about their land stability, or living in or next to homes already marked with white placards or red and yellow stickers, to be “extra cautious” throughout the cyclone event.
Landslide warning signs included new cracking or movement of the ground, sinking pavement, fresh debris or the sound of trees breaking.
“If you’re concerned about landslides, consider finding alternative accommodation, particularly during heavy rain.”
In the aftermath of last month’s historic flood event, and another atmospheric river event that followed days later, GNS scientists recorded hundreds of fresh slips, landslides and rockfalls across Auckland, mostly about hilly areas and coastal cliffs.
Another 160 landslides were documented in Tauranga alone, along with more than 450 across the Coromandel Peninsula.
In some of the most serious incidents, Remuera resident Dave Lennard was killed when a landslide crashed into his Shore Rd home on January 27, knocking it off its foundations.
Later, several homes in Tauranga’s Egret Ave were destroyed as a slip came down amid heavy rain, and another landslide that collapsed a seaside bach at Auckland’s Manukau Heads left three people injured.
As at last week, more than 270 Auckland houses – including multi-million-dollar luxury clifftop properties - had been red-stickered and more than 1600 yellow-stickered.
Niwa meteorologist Ben Noll said ground across most of the North Island was in a saturated state, with soil moisture observations showing levels tens of millimetres in surplus.
“We’re talking about ground that just hasn’t had a chance to dry out.”
University of Auckland coastal hazards researcher Professor Mark Dickson said Gabrielle raised the risk of further land-sliding along coastal cliff areas.
“The fact that we’ve already had a lot of rainfall leading up to this, can leave our coastal cliffs and beaches compromised to this storm.”
While many vulnerable sites would have “failed” already in the last big weather events – fresh slips could still occur in these places.
“It’s also possible that many new locations that didn’t fail in the last event may do this time around.”
Landslides at destabilised slopes and cliffs often happen amid heavy weather events, but can also occur weeks or months later.
“I work a lot on sea cliffs around Auckland, and almost by definition, these areas are prone to land-sliding failure” he said.
“The worry is that if you get an event that saturates the cliff, it’s going to make future failures more likely.”
Dickson and his colleagues were also trying to better understand the impact of wave action in major coastal storms like this.
“Where you get big waves attacking the toe of the cliff, this could steepen the face of the cliff upward, causing it to become more unstable,” he said.
“Over the long term, we think this undermining process is important – it’s unclear how much that’s likely to happen with this single storm, but it’ll be one of the things we’ll be looking at.”
Dickson’s team had already deployed sensors around Auckland’s coasts to measure wave height.
“The other thing that we should be expecting with this storm is considerable erosion of beaches,” he said, adding recent storms had left these natural coastal defence systems with significantly less sand.
“It is a concern that these beaches will be facing a storm of this kind with significantly depleted volumes of sand.”